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Causal inference and the Bradford Hill considerations
After chance, bias and confounding are excluded, whether an association is causal is weighed with the Bradford Hill considerations – and only one of them, temporality (the exposure precedes the disease), is a necessary condition.
Why it matters in the exam
Items on this topic ask you to identify the criterion described ("the more people smoked, the higher the risk" – dose–response), which finding does not support causation, or what is necessary to claim a causal link. Other items deal with Rothman's "pie" model – a sufficient cause made of several components – and with impact measures: attributable risk, the exposed group's attributable fraction and the population attributable risk, which are used to plan prevention. A typical calculation item gives the incidence in exposed and unexposed people and the prevalence of exposure, and asks what share of cases in the population would be prevented if the exposure were removed. Remember that even a weak association can be causal, and that specificity is a weak consideration because one exposure can have many effects. Another recurring point is the difference between additive and multiplicative interaction: when two risk factors together raise risk more than the sum of their effects, this has great practical meaning for prevention, because removing one factor also prevents the shared part. This also explains why the attributable fractions of several factors can add up to more than one hundred percent – each case has several causal components, and each is necessary within that pie.
Key points
- Causation is inferred only after chance, bias and confounding are excluded.
- Temporality is the only necessary consideration.
- Strength, dose–response, consistency and experimental evidence strengthen the claim.
- In Rothman's model a disease has several sufficient causes, each built of components.
- The population attributable risk combines the strength of association with the prevalence of exposure.
Summary table
| Consideration | Meaning | Exam note |
|---|---|---|
| Temporality | The exposure precedes the outcome | The only consideration that is a necessary condition |
| Strength | A high RR or OR | A strong association is less likely to be due to confounding alone, but a weak one can still be causal |
| Dose–response | More exposure – more disease | Strongly supportive; its absence does not exclude causation |
| Consistency | The finding recurs across populations and designs | Reduces the chance that it reflects chance or one study's bias |
| Plausibility and coherence | A mechanism exists, and the finding fits existing knowledge | Depends on the knowledge of the time |
| Experimental evidence and specificity | Removing the exposure lowers the risk; one exposure – one disease | An experiment is strongly supportive; specificity is a weak consideration |
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Sources
- Hill AB, The environment and disease: association or causation?, 1965 — Proceedings of the Royal Society of Medicine – original criteria
- Rothman, Epidemiology: An Introduction, 2nd ed., 2012 — What is causation?
- Gordis, Epidemiology, 6th ed., 2019 — From association to causation: deriving inferences
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.